Impedance plethysmograph

ABSTRACT

Plethysmograph having current and voltage electrodes with a variable current source connected to the current electrodes to supply varying current to a biological segment to provide a voltage which is utilized to provide a signal which represents a percent of change of resistance of the biological segment.

United States Patent 1 [111 3,882,851

Sigworth May 13, 1975 [54] IMPEDANCE PLETHYSMOGRAPH 3.566233 2/1971 Kahn128/2.1 Z 3,593,718 7 I971 K t 128 419 P 1 Inventor: Frederick Sigwmh,Ormda, Cahf- 3,641,993 21972 1282.] M

[73] Assignee: Systron-Donner Corporation.

Concord. Calif. 7 Primary Examiner-William E. Kamm [22] Flled 1973Attorney, Agent, or FirmFlehr, Hohbach, Test, [21] Appl. No.: 388,541Albritton & Herbert Related US. Application Data [63] Continuation ofSer. No. 190900, Oct. 20, 1971,

abandoned 57 ABSTRACT [52] US. Cl. 128/2.l Z; 128/205 V 51 Int. Cl A6lb5/04 Plethysmograph having Current and voltage electrodes 53 Field fSearch 12 2 5 p 105 R 205 v with a variable current source connected tothe curgg/zog 2 1 M, 2 1 p 2 1 Rs 21 Z, rent electrodes to supplyvarying current to a biologi- 145 5 4 4 9 p 419 R, '421 422 205 F, 2 calsegment to provide a voltage which is utilized to a L provide a signalwhich represents a percent of change of resistance of the biologicalsegment.

[56] References Cited UNITED STATES PATENTS 5 Claims, 4 Drawing Figures3,149,627 9/l964 Bagno l28/2.1 Z

17 control voltage 52? Auto-Null 36 37 I 55 7 TR R3 VARIABLE 5OKH VOSCILLATOR T3? 49 C so $53115 L 46'\- 41 SYNCHRONOUS DETECTOR a; Q, lR,'\:'

. Sync I! PATENTEBHAY 1 319. 5 3, 882.851

' sum 10F 3 r'-METER---u- NULL MODE OFF osv com) AUTO mm uAuxlo [IIII1I=IOpA seamen? Ion IOU-.0. IOOOIL Detected RESISTANCE F I g. 3

INVENTOR.

Frederick J. Sigworfh WJAZMAJW 414mg aw mimm I 3 I95 SHEET 3 OF 3INVENTOR.

BY Frederick J. Sigworfh w w Attorneys vim 1 IMPEDANCE PLETHYSMOGRAPHCROSS REFERENCES This is a continuation of application Ser. No. 190,900,filed Oct. 20, 1971, now abandoned.

BACKGROUND OF THE INVENTION In US. Pat. No. 3,149,627, there isdisclosed a plethysmograph which can be utilized for making impedancemeasurements on biological segments. A bridge arrangement is providedwhich must be manually nulled. Because the variations in resistance inthe biological segments are very small as, for example, on the order of0.1% or less, it is very difficult to null the meter because of drift.There is, therefore, a need for a new and improved plethysmograph.

SUMMARY OF THE INVENTION AND OBJECTS The plethysmograph consists offirs: and second current electrodes and first and second voltageelectrodes adapted to be applied in pairs across the biological segmentwith one current electrode adjacent one voltage electrode and the othercurrent electrode adjacent the other voltage electrode. A high frequencyoscillator is provided. A variable current source is connected to theoutput of the oscillator and to the current electrodes so that anoscillatory current is supplied to the current electrodes. Amplifiermeans is coupled to the voltage electrodes for measuring the differencevoltage developed across the voltage electrodes. Means is coupled to theamplifier and to the high frequency oscillator for detecting the outputof the amplifier using the output of the oscillator as a reference.Means is provided for supplying the output of the detector means toprovide a control voltage for the variable current source toautomatically null the same. The output of the detector produces asignal which represents a percent of change of the resistance of thebiological segment.

In general, it is an object of the present invention to provide animpedance plethysmograph which is quite accurate and reliable.

Another object of the invention is to provide a plethysmograph of theabove character which is relatively easy to operate.

Another object of the invention is to provide a plethysmograph which canprovide a readout in either percent of deviation or in conductance.

Another object of the invention is to provide a plethysmograph of theabove character in which an automatic nulling function is provided.

Another object of the invention is to provide a piethysmograph of theabove character which can be battery operated.

Another object of the invention is to provide a plethysmograph of theabove character in which the readout is in percent variation rather thanan absolute resistance indication.

Additional objects and features of the invention will appear from thefollowing description in which the pre-- ferred embodiment is set forthin detail in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a perspective view of animpedance plethysmograph incorporating the present invention connectedto a body segment.

FIG. 2 is a block diagram of the impedance plethysmograph.

FIG. 3 is a graph showing current flow for various values of resistance.

FIG. 4 is a detailed circuit diagram of the variable current source inFIG. 2.

DESCRIPTION OF PREFERRED EMBODIMENT The impedance plethysmographincorporating the present invention is in the form of an electronicinstrument 11 as shown in FIG. 1 which includes a case 12. The case 12is provided with a stand 13 so that the front panel 16 of the instrumentcan be elevated above the top surface of a table or other supportprovided for the instrument. A meter 17 is mounted in the front paneland is capable of measuring percent deviation according to the deviationscaling selected as hereinafter described. It is also capable of reaingmillimhos according to the scaling mode selected as hereinafterdescribed. The front panel is also provided with a connector 18 which isconnected to a cable 19. The cable 19 is provided with four conductorswhich have been identified as first and second current electrodes 1 Iand first and second voltage electrodes E E shown in FIG. 1 and whichare preferably color coded in a suitable manner. For example, theconductors I and I can be color coded red, whereas the conductors E andE can be color coded black. Each of the conductors which are provided,1,, l and E E is connected to relatively thin metallic strips 21 formedof a suitable material such as aluminum. These strips serve aselectrodes which are secured to the biological segment L of which themeasurements are to be made such as a human body segment in the form ofa finger 22 by suitable means such as an adhesive tape (not shown). Theelectrodes are arranged in pairs across the body segment L with onecurrent electrode adjacent one voltage electrode and the other currentelectrode adjacent the other voltage electrode. Thus, electrodes l and Eare adjacent to each other and the electrodes E and I are adjacent toeach other.

A total of 12 pushbuttons 26 are arranged in two spaced parallel rows,the function of which will hereinafter be described. Three output jacksare provided as a part of the instrument in which one of the jacks 31 isprovided on the front panel. These jacks are simultaneously active. Thefront jack 31 provides deviation, that is, recording resistance changerelated to blood or gas volume variation. The two rear jacks (not shown)provide a derivative signal which is the rate of change of the deviationsignal and a conductance signal which monitors the relatively slowchanges in the conductance of the examined segment during change inposture or in response to vasodilation or vasoconstriction. A referencepotentiometer 32 is mounted on the front panel. In addition, two lights33 and 34 are provided. Light 33 indicates that the instrument is onbattery operation and light 34 indicates that the battery is beingcharged.

A block diagram of the electronic circuitry within the case 12 is shownin FIG. 2. As shown therein, it consists of an oscillator 36 operatingat a suitable frequency as, for example, 50 kilohertz (KHz). Theoscillator 36 supplies a timing signal through a conductor 38 to avariable current source 37. The variable current source supplies onoutput line 39 an output current which is related in amplitude to thecontrol voltage which is supplied to the current source. This outputcurrent is identified by the letter I and is fed into the body segment Lwhich is represented by the resistance R consisting of three resistors RR and R in series. R and R represent the contact resistance between theelectrodes 1, and I and the skin of the body member to which theelectrodes are secured. Two taps 41 and 42 are provided on oppositesidesof the resistor R and correspond to the two electrodes connected tothe conductors E and E The conductors E and E are connected to adifferential amplifier 43 which measures the difference voltagedeveloped across the body segment resistance R and is indicated as V Theoutput of the differential amplifier 43 is supplied to a synchronousdetector 44. The detector 44 has an output 46 which is connected to aterminal 47 of a switch 48 that has an additional terminal 49. Theswitch 48 is connected to the meter 17. When the switch 48 is connectedto the terminal 47, it will be reading the resistance changes in thebody segment itself which is supplied by the synchronous detector. Whenthe switch is connected to the terminal 49 which is connected to thevariable current source 37 by conductor 51, the meter will be readingthe conductance of the body segment itself by measuring the amount ofcurrent that the current source is putting into the body segment. Theoutput of the synchronous detector is also supplied to the variablecurrent source by a conductor 52 to provide a control voltage ashereinafter described.

A referance voltage is supplied to the synchronous detector 44 throughthe conductor 56 which is connected between two serially connectedresistors R4 and R5 that form a voltage divider network connectedbetween ground and the oscillator 36. A synchronizing voltage issupplied to the synchronous detector 44 from the oscillator 36 by aconductor 57.

By way of example, the reference voltage supplied to the synchronousdetector can be on the order of 100 millivolts. If the voltage suppliedby the differential amplifier is greater than 100 millivolts, the outputof the synchronous detector is positive and if the voltage from thedifferential amplifier is less than 100 millivolts, then the output fromthe detector 44 is negative. Thus, the detector 44 provides a dc.voltage which gives information as to whether or not the output of thedifferential amplifier is above or below the null point. Thus, there issupplied a control voltage to the variable current source which performsan automatic nulling function. Thus, when the output from thesynchronous detector 46 is negative, the variable current source 47 willbe instructed to supply an additional voltage across the body segment toincrease the current flow through the body segment. Conversely, if theoutput of the synchronous detector 44 is positive, the variable currentsource will be instructed to supply a lesser voltage to the body segmentto thereby decrease the current flow through the body segment. It can beseen that this is similar to nulling a bridge where the differentialsignal voltage developed across the body segment is compared with aninternal reference voltage. In the present nulling system, the currentsupplied to the body segment itself is varied which means that thevoltage level developed across the body segment is a long term constant,although short-term small changes in the body segment resistance willshow up as variations superimposed on the 100 millivolt output from thedifferential amplifier and, therefore, will show up as instantaneousvariations in the output of the synchronous detector 44.

The control of the current passing through the body segment L to providea nominally constant voltage makes it possible for the output read onthe meter 17 to represent the percent of value change rather thanabsolute resistance. Thus, for example, if the body segment resistanceis ohms, it would develop a 10 millivolt signal across the body segmentwhich requires 0.1 of a milliampere to pass through the body segmentresistance. The 10 millivolts across the body segment is picked off bythe conductors E and E and amplified as, for example, ten times in thedifferential amplifier 43 to provide the 100 millivolt output to thesynchronous detector 44. The relatively slow response time of thecontrol voltage ensures that the current level will remain constant forshort-term variations. If there is an instantaneous change of resistancein the body segment, as for example, by 0.] ohm, this would mean achange in the signal voltage measured across the body segment of the 0.1ohm times a 0.1 of a milliampere or 10 microvolts of change at theoutput of the synchronous detector 44. The synchronous detector 44 wouldthen amplify by a suitable gain such as 1,000 to provide 100 millivoltsof signal which is supplied to the meter 17 which would show a variationof 0. l i.e., 0.1 of an ohm out of 100 ohm.

If the segment resistance is 50 ohms instead of 100 ohms, then in orderto obtain 100 millivolts at the input to the detector or 10 millivoltsat the input to the differential amplifier,, the feedback loop wouldcause the current source to supply 0.2 of a milliampere of current tothe body segment. Then if there is 0.1% change in the resistance of 50ohms which would be 0.05 ohms, the current level would be double whichwould provide the same 10 microvolts of signal being supplied to thedifferential amplifier and thus eventually would result in the same 100millivolt signal to the meter 17.

In FIG. 3, there is a curve which shows the current and power involvedin the measurement process with current and power variations. The X-axisshows body segment resistances from 10 ohms to l k. The Y-axis showsboth the power and current involved in the body segment at eachresistance. A logarithmic scale is used in both axes. As shown in thegraph, there were 100 microamperes of current flowing through the bodysegment when the body segment had a resistance of 100 ohms. If the bodysegment resistance instead of being 100 ohms is 10 ohms, the feedbackloop would change the variable current source 37 until the current levelwould be 1 milliampere and the power would be 10 microwatts.

The chart shows that the current level is inversely proportional to thebody segment resistance. It also shows that only very small current andpower levels are required for making the desired measurements, so smallthat no sensation is felt or health hazard is encountered by use of thedevice.

Of the blocks shown in FIG. 2, the oscillator 36, the differentialamplifier 43 and the synchronous detector 44 are of a conventional type.By way of example, the synchronous detector 44 can be like the phasesensitive detector 15 disclosed in U.S. Pat. No. 3,149,627.

A detailed circuit diagram of the variable current source 37 is shown inFIG. 4. As shown therein, the control voltage from the detector 44 issupplied through the lead 52 to an integrator 62 consisting of anamplifier 63, a resistor 61 and a capacitor 64 which is connectedbetween the input and the output of the amplifier 63. The output of theintegrator 62 is supplied to a log function generator 66 of a type wellknown to those skilled in the art which consists of a plurality of 5serially connected diodes 67 and a plurality of serially connectedresistors 68 with one of the resistors 68 being connected across each ofthe diodes 67. The log function generator 66 produces a voltage acrossthe same which is the logarithm of the current flowing through the logfunction generator. The use of the log function generator makes itpossible for the system to have a constant response time regardless ofthe body segment resistance and the current level that is fed into thesegment L. Transistors Q1, Q2, Q3 and Q4 are provided in the circuitshown in FIG. 4. They are all of a conventional type and each includesbase, collector and emitter elements as shown.

The 50 KHZ signal from the oscillator 36 is fed onto the line 38 througha resistor 71 to the base of a transistor Q1 which serves as a switchingtransistor for switching the signal supplied by the log functiongenerator away from the transistor Q2 for nominally half of the time.The other half of the time O1 is turned off under the control of thesignal supplied on the line 38 and all of the current through thefunction generator is supplied to the emitter of the transistor Q2. Theresult is that a square wave current output is supplied by the collectorof Q2.

The square wave output from the collector of the transistor O2 issupplied to a tuned circuit 76 which is resonant at the same frequencyas the oscillator 36 and consists of an inductance 77 and a capacitor 78connected across the inductance. The tuned circuit 76 converts thesquare wave into a uniform sine wave. The current exciting the tunedcircuit 76 is also supplied through a sampling resistor 79 for theconductance readout by the meter 17. A capacitor 81 connected across theresistor 79 serves as a bypass capacitor. The resistor 79 is connectedto a 6 volt source as indicated in FIG. 4. The resistor 79 measures theaverage value of current flowing through the log function generator 66and through the transistor Q2 and through the inductance 77. Thiscurrent is proportional to the output current from the entire currentsource 37. This current is supplied through a resistor 82 to anoperational amplifier 83 which has a resistor 84 connected between theinput and the output of the amplifier 83. The output of the amplifier isconnected to a terminal which is identified as l/R which is adapted tobe connected to the meter 17 to provide a conductance readout on themeter 17.

The 50 KHZ sine wave signal developed across the tuned circuit 76 isalso supplied through a capacitor 86 to the base of a current sourcetransistor Q3. The base is also connected to ground through a resistor87. The emitter of the transistor is connected to a +6 volt supplyvoltage through a resistor 88.

The collector of the transistor O3 is connected through a resistor 89 tothe base of a transistor Q4. The base of O4 is also connected through acapacitor 91 and a resistor 94 to a -6 volt source as indicated in FIG.4. The emitter of the transistor O4 is connected through a resistor 92to the 6 volt source. The collector is coupled through a capacitor 93 tothe current output line 39 which is connected to the resistor R as shownin FIG. 2.

The transistor 04 and its associated components, capacitors 91 and 93,serve as a dc. current source to provide operating bias for thetransistor Q3. The output impedance of the current source is high,limited only by the collector impedances of Q3 and Q4 and the value ofresistor 89.

The instrument is provided with a battery which is not shown. Means isalso provided in the instrument for charging the battery when theinstrument is not in use. Circuitry is provided which is associated withthe pushbuttons 26 on the front panel so that switching from the offbutton to either the conductance or the deviation button disconnects theinstrument from the a.c. power supply and switches to battery power. Thebattery operating light 33 will turn on if the battery is operational.When the instrument is not in use, it can be connected to an a.c. poweroutlet. Pushing the off button will activate the battery chargingcircuit and the light 34 will be lit.

Additional circuitry is provided which is connected to the otherpushbuttons which are utilized in connection with the operation of theinstrument as hereinafter described.

Let it be assumed that it is desired to make an impedance measurement ofa body segment as, for example, of one finger as shown in FIG. 1. Theelectrodes are applied by adhesive tape as hereinbefore described andpositioned in the manner shown in FIG. 1.

Now let it be assumed that it is desired to operate the instrument inthe manual balancing mode. If the segmental resistance (E E is belowohms, the deviation button, the manual button and the 20% deviation modebutton should be pushed. The reference potentiometer 32 is adjusted fora zero center reading on the meter 17. Alternatively, the 2% and 0.2%buttons can be pushed to obtain finer tuning by the use of the referencepotentiometer 32.

When the segmental resistance is above I00 ohms, the manual X 10 buttonis pushed and the reference potentiometer is switched from zero to a1,000 ohm scaling. The deviation can be read directly from the meter 17according to the scaling (20%, 2%, 0.2%), or the deviation may bedisplayed on an oscilloscope or recorded on a laboratory recorder byconnecting the same to the deviation jack 31 provided on the frontpanel. When the conductance button is pushed, the conductance inmillimhos can be read on the meter. The reference potentiometer 32 readsdirectly in ohms. Thus, in a manual mode, the potentiometer 32 readsfrom zero to 100 ohms. In the manual X 10 mode, the potentiometer 32reads zero to 1,000 ohms. The reading on the reference potentiometer 32corresponds with the conductance reading on the meter 17.

Let it be assumed that automatic null operation is desired. When this isthe case, the deviation button, the automatic null button and one of thedeviation scaling buttons (20%, 2%, 0.2%) should be pushed. Thedeviation can be read directly from the meter 17 or displayed on alaboratory recorder from the output jack 31 as hereinbefore described.When the conductance button is pushed, the conductance can be readdirectly off the meter or displayed on a laboratory recorder from theoutput jack provided on the back side of the instrument. Nulling isperformed automatically in the automatic mode and the referencepotentiometer is not used in the automatic null configuration.

In practice, it may be desirable to determine the resistance from thereference potentiometer 32 by using the instrument either in manual ormanual X 10 mode and then switching to the automatic null mode to obtainconductance.

During these measurements, it should be appreciated that since theinstrument is being operated from a battery, there is no possibility ofsupplying voltage to the patient. In addition, it should be appreciatedthat the voltage and currents which are being applied to the biologicalsegments of the patient are so -low that they will not cause any harm.

In the diagram shown in FIG. 2, the meter 17 is connected to l/Rterminal which corresponds to the condition when the conductance buttonis pushed. The meter 17 registers the body segment conductance which isreciprocal of the resistance. Thus, when automatic nulling is takingplace, the current passing through the body segment is inverselyproportional to the segment resistance. Therefore, the current suppliedinto the body segment is proportional to the segment conductance. Themonitoring voltage which is supplied at the l/R, terminal shows theamount of current which is being supplied to the body segment which canbe read out directly on the meter 17. The meter 17 is provided with twoscales. The meter normally rests in the center of the two scales. Onescale is in deviation and provides deviation in percent in bothdirections. The second scale is calibrated in millimhos of conductancefrom zero to 10 or zero to I00 millimhos depending upon which pushbuttonis pressed.

The measurements hereinbefore described by the use of the four electrodeimpedance plethysmograph makes it possible to measure changes inelectrical resistance related to blood and gas volumes in biologicalsegments. During each cardiac cycle as blood is distributed from theheart to various organs and tissues, the new blood volume results in achange in resistance which increases current conduction from theplethysmograph and results in a pulsatile voltage output which can bedetected and recorded for qualitative and quantitative evaluation.

In a similar manner, increases and decreases in gas volume duringrespiratory activity result in variations in electrical resistance ofthe thoracic segment which can be detected and recorded as an index ofpulmonary function.

The high frequency current from the plethysmograph penetrates throughsuperficial and deep body structures, and each segment defined by theplacement of electrodes may be interpreted as a homogeneous volumeconductor having a measure of resistance referred to as the base lineresistance.

This measured segmental resistance is influenced by the pulsatile, gasor blood volume variations as well as increases or decreases in totalblood or body tissue fluid within the segment which may occur duringvasoconstriction, vasodilation or postural variation. The impedanceplethysmograph is capable of recording both the pulsatile changes(deviation) and variations in base line resistance which is registeredas the reciprocal conductance.

In summary, the basic principle utilized by the impedance plethysmographdepends upon changes in the electrical characteristics of tissue duringeach cardiac or respiratory cycle as a new volume of blood or gas entersthe segment. In terms of pulsatile blood volume changes, the conductiveproperties of the segment and the new volume of blood parallel eachother during systole and create a change in resistance which isproportional to the pulse volume. Variations in resistance occur in thethoracic cavity during respiration as a result of the difference betweenblood and gas volumes in the chest during respiratory activity.

The impedance plethysmograph offers the clinician, medical specialist,educator or biologist and researcher a reliable, easy to use and highlyversatile electronic instrument. The plethysmograph output can beconnected to oscillographs and pen recorders for simultaneous recordingof ECG and changes in blood pulse volume and respiratory volume ofbiological segments. The use of the impedance plethysmograph does notcause trauma to the patient. The system does not require venousocclusion or methods for detecting small and large changes in electricalresistance related to blood pulse volume variations andis useful indefining pulmonary gas volume when electrodes are positioned over thethorax area.

The impedance plethysmograph can be useful in medical schools forteaching principles of cardiovascular and pulmonary physiology, inhospitals for the management of patients preand post-operatively, inanimal and clinical diagnostic laboratories for drug evaluation, and inoffices of the general practicioner for early detection ofcardio-vascular disorders. The deviation output (AR) is located on thefront and rear panels. The base line (conductance) and the derivatived(AR/R)/d, outputs are available at the rear panel for simultaneousrecordings. The selector pushbuttons activate the meter for additionalmonitoring of all three simultaneous recorder outputs.

It is apparent from the foregoing that there has been provided animpedance plethysmograph which has many desirable features. It isportable and it is simple to operate. It can be operated in eithermanual balancing or automatic balancing modes.

The incorporation of a constant current source of variable output as anulling element in the impedance plethysmograph is particularlyimportant because it makes it possible to provide a readout which is apercent of the percent variation rather than an absolute resistanceindication on a meter.

I claim:

1. In a plethysmograph first and second current input electrodes, firstand second voltage drop measuring electrodes adapted to be applied inpairs across a biological segment with one current electrode adjacentone voltage electrode and the other current electrode adjacent the othervoltage electrode, an oscillator having a high frequency output, avariable current source connected to the oscillator and to the first andsecond current input electrodes for receiving a timing signal from theoscillator so that an oscillatory output current is provided to thecurrent input electrodes, amplifier means coupled to said first andsecond voltage drop electrodes for receiving the voltage developedacross said voltage drop electrodes and producing an output responsivethereto, means for providing a predetermined reference voltage, meanscoupled to the amplifier means for detecting the output of the amplifiermeans and providing an output responsive thereto, said means fordetecting receiving said predetermined reference voltage, means forsupplying the output of the means for detecting to the variable currentsource to provide a control voltage for the variable current source sothat the output from the variable current source provides an output fromthe amplifier means matching said predetermined reference voltage, theoutput of said means for detecting providing a percent deviation signalwhich represents a percent of change of resistance value of thebiological segment, indicating means for receiving the percent deviationsignal from the means for detecting for indicating the percent of changeof resistance, said indicating means also being for indicatingconductance of the biological segment, and means for alternatelyconnecting said indicating means for receiving the output of thevariable current source.

2. In a plethysmograph first and second current input electrodes, firstand second voltage drop measuring electrodes adapted to be applied inpairs across a biological segment with one current electrode adjacentone voltage electrode and the other current electrode being adjacent theother voltage electrode, an oscillator having a high frequency output, avariable current source connected to the oscillator and to the first andsecond current input electrodes for receiving a timing signal from theoscillator so that an output oscillatory current is provided to thecurrent input electrodes, amplifier means coupled to said first andsecond voltage drop electrodes for receiving the voltage developedacross said voltage drop electrodes and producing an output responsivethereto, means for providing a predetermined reference voltage, meanscoupled to the amplifier means for detecting the output of the amplifiermeans and providing an output responsive thereto, said means fordetecting receiving said predetermined reference voltage, means forsupplying the output of the means for detecting to the variable currentsource to provide a control voltage for the variable current source sothat the output from the variable current source provides an output fromthe amplifier means matching said predetermined reference voltage, theoutput of the means for detecting providing a percent deviation signalwhich represents a percent of change of resistance value of thebiological segment, and indicating means calibrated in terms of twoquantities, one of said quantities being conductance and the other ofsaid quantities being percent of resistance change, together with meansfor connecting said indicating means alternately to said variablecurrent source and said means for detecting respectively.

3. In a plethysmograph first and second current input electrodes, firstand second voltage drop measuring electrodes adapted to be applied inpairs across a biological segment with one current electrode adjacentone voltage electrode and the other current electrode being adjacent theother voltage electrode, an oscillator having a high frequency output, avariable current source connected to the oscillator and to the first andsecond current input electrodes for receiving a timing signal from theoscillator so that an output oscillatory current is provided to thecurrent input electrodes, amplifier means coupled to said first andsecond voltage drop electrodes for receiving the voltage developedacross said voltage drop electrodes and producing an output responsivethereto, means for providing a predetermined reference voltage, meanscoupled to the amplifier means for detecting the output of the amplifiermeans and providing an output responsive thereto, said means fordetecting receiving said predetermined reference voltage, means forsupplying the output of the means for detecting to the variable currentsource to provide a control voltage for the variable current source sothat the output from the variable current source provides an output fromthe amplifier means matching said predetermined reference voltage,wherein said current source includes a logarithmic function generatorelectrically coupled between the control voltage input and theoscillatory current output so that the plethysmograph will have aconstant response time regardless of the biological segment resistanceand the current level supplied to the biological segment, the output ofthe means for detecting providing a percent deviation signal whichrepresents a percent of change of resistance value of the biologicalsegment, and indicating means for receiving the percent deviation signalfrom the means for detecting and for indicating the percent change ofresistance.

4. In a plethysmograph, first and second current input electrodes, firstand second voltage drop measuring electrodes adapted to be applied inpairs across a biological segment with one current electrode adjacentone voltage electrode and the other current electrode being adjacent theother voltage electrode, an oscillator having a high frequency output, avariable current source connected to the oscillator and to the first andsecond current input electrodes for receiving a timing signal from thecurrent input electrodes, said variable current source including atleast two active electronic amplification means connected in series andproviding an output impedance which is high with respect to theimpedance of the biological segment so that plethysmograph calibrationwill not be substantially affected by changes in the impedance of thebiological segment, amplifier means coupled to said first and secondvoltage drop electrodes for receiving voltage developed across saidvoltage drop electrodes and producing an output responsive thereto,means for providing a predetermined reference voltage, detecting meanscoupled to the amplifier means and to the means for providing apredetermined reference voltage for detecting the output of theamplifier means and providing an output responsive thereto, means forsupplying the output of the detecting means to the variable currentsource to provide a control voltage for the variable current source sothat the output from the variable current source develops a voltageacross said first and second voltage drop electrodes which providesoutput from the amplifier means automatically nulling said predeterminedreference voltage, the output of the detecting means providing a percentdeviation signal which represents a percent of change of the value ofresistance of the biological segment, a plurality of serially connecteddiodes and a plurality of serially connected resistors with one of saidresistors connected across each of said diodes included in said variablecurrent source for providing a voltage thereacross which is thelogarithm of the current therethrough, whereby a constant systemresponse time is obtained regardless of biological segment resistance orcurrent source output level,

and indicating means for receiving the percent deviation signal from themeans for detecting and for indicating the percent of change ofresistance.

5. In a plethysmograph, first and second current input electrodes, firstand second voltage drop measuring electrodes adapted to be applied inpairs across a biological segment with one current electrode adjacentone voltage electrode and the other current electrode being adjacent theother voltage electrode, an oscillator having a high frequency output, avariable current source connected to the oscillator and to the first andsecond current input electrodes for receiving a timing signal from theoscillator so that an oscillator output current is provided to thecurrent input electrodes, amplifier means coupled to the voltage dropelectrodes for receiving the voltage developed across the voltageelectrodes and producing an output responsive thereto, means coupled tothe amplifier means for detecting the output of the amplifier means andproviding an output responsive thereto, means for supplying the outputof the means for detecting to the variable current source to provide acontrol voltage for the variable current source so that the variablecurrent source provides an output producing a long term constant voltageacross the first and second voltage drop electrodes, said variablecurrent source including a logarithmic function generator for providinga constant output response time over a range of biological segmentresistance values covering substantially two orders of magnitude, theoutput of the means for detecting providing a percent deviation signalwhich represents a percent of change of resistance value of thebiological segment, and means connected to receive the percent deviationsignal for indicating the biological segment percent change ofresistance.

1. In a plethysmograph first and second current input electrodes, firstand second voltage drop measuring electrodes adapted to be applied inpairs across a biological segment with one current electrode adjacentone volTage electrode and the other current electrode adjacent the othervoltage electrode, an oscillator having a high frequency output, avariable current source connected to the oscillator and to the first andsecond current input electrodes for receiving a timing signal from theoscillator so that an oscillatory output current is provided to thecurrent input electrodes, amplifier means coupled to said first andsecond voltage drop electrodes for receiving the voltage developedacross said voltage drop electrodes and producing an output responsivethereto, means for providing a predetermined reference voltage, meanscoupled to the amplifier means for detecting the output of the amplifiermeans and providing an output responsive thereto, said means fordetecting receiving said predetermined reference voltage, means forsupplying the output of the means for detecting to the variable currentsource to provide a control voltage for the variable current source sothat the output from the variable current source provides an output fromthe amplifier means matching said predetermined reference voltage, theoutput of said means for detecting providing a percent deviation signalwhich represents a percent of change of resistance value of thebiological segment, indicating means for receiving the percent deviationsignal from the means for detecting for indicating the percent of changeof resistance, said indicating means also being for indicatingconductance of the biological segment, and means for alternatelyconnecting said indicating means for receiving the output of thevariable current source.
 2. In a plethysmograph first and second currentinput electrodes, first and second voltage drop measuring electrodesadapted to be applied in pairs across a biological segment with onecurrent electrode adjacent one voltage electrode and the other currentelectrode being adjacent the other voltage electrode, an oscillatorhaving a high frequency output, a variable current source connected tothe oscillator and to the first and second current input electrodes forreceiving a timing signal from the oscillator so that an outputoscillatory current is provided to the current input electrodes,amplifier means coupled to said first and second voltage drop electrodesfor receiving the voltage developed across said voltage drop electrodesand producing an output responsive thereto, means for providing apredetermined reference voltage, means coupled to the amplifier meansfor detecting the output of the amplifier means and providing an outputresponsive thereto, said means for detecting receiving saidpredetermined reference voltage, means for supplying the output of themeans for detecting to the variable current source to provide a controlvoltage for the variable current source so that the output from thevariable current source provides an output from the amplifier meansmatching said predetermined reference voltage, the output of the meansfor detecting providing a percent deviation signal which represents apercent of change of resistance value of the biological segment, andindicating means calibrated in terms of two quantities, one of saidquantities being conductance and the other of said quantities beingpercent of resistance change, together with means for connecting saidindicating means alternately to said variable current source and saidmeans for detecting respectively.
 3. In a plethysmograph first andsecond current input electrodes, first and second voltage drop measuringelectrodes adapted to be applied in pairs across a biological segmentwith one current electrode adjacent one voltage electrode and the othercurrent electrode being adjacent the other voltage electrode, anoscillator having a high frequency output, a variable current sourceconnected to the oscillator and to the first and second current inputelectrodes for receiving a timing signal from the oscillator so that anoutput oscillatory current is provided to the current input electrodes,amplifier means coupled tO said first and second voltage drop electrodesfor receiving the voltage developed across said voltage drop electrodesand producing an output responsive thereto, means for providing apredetermined reference voltage, means coupled to the amplifier meansfor detecting the output of the amplifier means and providing an outputresponsive thereto, said means for detecting receiving saidpredetermined reference voltage, means for supplying the output of themeans for detecting to the variable current source to provide a controlvoltage for the variable current source so that the output from thevariable current source provides an output from the amplifier meansmatching said predetermined reference voltage, wherein said currentsource includes a logarithmic function generator electrically coupledbetween the control voltage input and the oscillatory current output sothat the plethysmograph will have a constant response time regardless ofthe biological segment resistance and the current level supplied to thebiological segment, the output of the means for detecting providing apercent deviation signal which represents a percent of change ofresistance value of the biological segment, and indicating means forreceiving the percent deviation signal from the means for detecting andfor indicating the percent change of resistance.
 4. In a plethysmograph,first and second current input electrodes, first and second voltage dropmeasuring electrodes adapted to be applied in pairs across a biologicalsegment with one current electrode adjacent one voltage electrode andthe other current electrode being adjacent the other voltage electrode,an oscillator having a high frequency output, a variable current sourceconnected to the oscillator and to the first and second current inputelectrodes for receiving a timing signal from the current inputelectrodes, said variable current source including at least two activeelectronic amplification means connected in series and providing anoutput impedance which is high with respect to the impedance of thebiological segment so that plethysmograph calibration will not besubstantially affected by changes in the impedance of the biologicalsegment, amplifier means coupled to said first and second voltage dropelectrodes for receiving voltage developed across said voltage dropelectrodes and producing an output responsive thereto, means forproviding a predetermined reference voltage, detecting means coupled tothe amplifier means and to the means for providing a predeterminedreference voltage for detecting the output of the amplifier means andproviding an output responsive thereto, means for supplying the outputof the detecting means to the variable current source to provide acontrol voltage for the variable current source so that the output fromthe variable current source develops a voltage across said first andsecond voltage drop electrodes which provides output from the amplifiermeans automatically nulling said predetermined reference voltage, theoutput of the detecting means providing a percent deviation signal whichrepresents a percent of change of the value of resistance of thebiological segment, a plurality of serially connected diodes and aplurality of serially connected resistors with one of said resistorsconnected across each of said diodes included in said variable currentsource for providing a voltage thereacross which is the logarithm of thecurrent therethrough, whereby a constant system response time isobtained regardless of biological segment resistance or current sourceoutput level, and indicating means for receiving the percent deviationsignal from the means for detecting and for indicating the percent ofchange of resistance.
 5. In a plethysmograph, first and second currentinput electrodes, first and second voltage drop measuring electrodesadapted to be applied in pairs across a biological segment with onecurrent electrode adjacent one voltage electrode and the other currentelectrode being adjaceNt the other voltage electrode, an oscillatorhaving a high frequency output, a variable current source connected tothe oscillator and to the first and second current input electrodes forreceiving a timing signal from the oscillator so that an oscillatoroutput current is provided to the current input electrodes, amplifiermeans coupled to the voltage drop electrodes for receiving the voltagedeveloped across the voltage electrodes and producing an outputresponsive thereto, means coupled to the amplifier means for detectingthe output of the amplifier means and providing an output responsivethereto, means for supplying the output of the means for detecting tothe variable current source to provide a control voltage for thevariable current source so that the variable current source provides anoutput producing a long term constant voltage across the first andsecond voltage drop electrodes, said variable current source including alogarithmic function generator for providing a constant output responsetime over a range of biological segment resistance values coveringsubstantially two orders of magnitude, the output of the means fordetecting providing a percent deviation signal which represents apercent of change of resistance value of the biological segment, andmeans connected to receive the percent deviation signal for indicatingthe biological segment percent change of resistance.